1. Field of Invention
The present invention relates to a field of USB communication, and more particularly to a circuit for generating a peripheral clock for USB and a method therefor.
2. Description of Related Arts
USB is an abbreviation of Universal Serial BUS. USB has advantages of high speed, simple port, convenient application, etc. Thus, nowadays, USB is one of necessary standard ports of electronic equipments, such as PC (Personal Computer), MP4 (Mobile Pentium 4), mobile phone, PDA (Personal Digital Assistant), digital camera, printer, and scanner, and the USB is widely applied in aspects of info-communication and data transmission.
A conventional communicating system usually requires an accurate clock source. Based on the clock source, a master clock necessary for the communicating system is generated in the communicating system by logic, such as frequency division and frequency multiplication. The master clock is for analyzing and collecting data, and thus data communication is achieved. USB communicating system is no exception. Full speed communication has a communicating speed of 12 MHz and low speed communication has a communicating speed of 1.5 MHz, wherein USB communicating system requires a high accuracy of master clock for transmitting data, i.e., ±2.5% o. Therefore, USB communicating system usually adopts a clock solution of peripheral crystal oscillator, i.e., the peripheral crystal oscillator generates an accurate clock, for example 12 MHz, to be input in a chip. The chip generates a working clock of 300 MHz by logic module for frequency multiplication, such as PLL, wherein the working clock of 300 MHz is required by the USB communicating system at a low speed or a full speed, in order to guarantee communicating accuracy of USB major structure.
Because the crystal oscillator is required to provide the accurate clock for the USB major structure, the chip of USB communicating system should provide two extra pins for the crystal oscillator. Thus, USB communicating system is not applicable for electronic products which have few pins. Conventional electronic products, such as subscriber identity module, usually have 4˜7 pins for other system functions, and there is no extra pins for the crystal oscillator. Therefore, the USB communicating system is not applicable for these electronic products.
In addition, with development of craftsmanship and design technique, volume of the electronic products become smaller and smaller, and a number of pins of electronic products is reduced. Volume of crystal oscillator is relatively big for SOC (System on Chip), which restricts highly integration and miniaturization of the electronic products. Therefore, the crystal oscillator becomes a critical factor to restrict application and development of the chip of USB communicating system.
The clock can also be generated by RC/LC oscillator in the chip of USB communicating system, and be input to the USB major structure. However, the clock generated by the RC/LC oscillator in the chip usually deviates from design objective by ±20%, because of process variations of RC/LC oscillator or other factors. Deviation will cause inconformity of lengths of packets in sending and receiving, which can not meet a demand of communicating accuracy of system.
Therefore, it is necessary to provide a circuit for generating a peripheral clock for USB and a method therefor, to overcome the above defects.